European academies measured the solar system by timing Venus transits worldwide

By the early 18th century, astronomers agreed on the relative distances between the planets — Kepler's laws gave the ratios precisely — but had no reliable measurement of the absolute distance from Earth to the Sun, the astronomical unit, which anchored the entire scale of the solar system.

In 1716, astronomer Edmond Halley published a method exploiting a rare event: when Venus passes directly between Earth and the Sun, its silhouette crosses the Sun's disc at a slightly different position and timing depending on where on Earth the observer stands, because Venus is close enough to Earth for that parallax to be detectable, unlike the Sun itself.

Sent observers worldwide to time Venus transits, using the parallax difference between separated sites as the signal—a measurement no single observatory could produce.

The Sun's distance is defined by parallax, which only appears as a difference between two observers.

No single observatory could see that difference, so improving instruments alone could never reveal it.

Timing the Venus transit from many separated points created the needed baseline.

Combining those timings mathematically turned local data into a global measurement.

When a quantity only exists as a difference across vantage points, build the measurement from separation itself, not from a sharper single instrument.

Despite observational difficulties from the 'black drop effect' that limited precise timing, the combined international data let French astronomer Jérôme Lalande calculate the Earth-Sun distance at roughly 153 million kilometers in 1771 — within about 1% of the modern measured value of 149.6 million kilometers, and vastly more accurate than any prior estimate, some of which had been off by a factor of ten or more. The method was refined further using additional transit data from 1874 and 1882 before later superseded by radar measurement in the 20th century.